MyCrisis

Drove past a radar gun speed sign ("Your speed is XX") the police set up.

Pegged the speedo at 30, exactly. Radar gun read 27 mph. Did it 3 times for multiple trials, and each showed the same discrepancy. Then got my other car and drove by - speedo matched the radar gun exactly.

I run 205/40/R17, but this should be same diameter as stock. What gives? Anybody else notice a difference in 2004 models?

Does this also mean my odometer is off, that I've more/less miles than the odometer reads, and my MPG is better/worse than I figure?

draxcaliber

yes, if your speedometer is off, then your odo is off and your so will your mph calculations.

keep in mind however that there is a variation in what the radar detector will read because it is measuring your speed from an angle, not dead on, something call the sin or cos error, so the radar detector isn't 100% correct either.

MyCrisis

So whadda think - is the odo reading too high (showing more miles than I have) or too low (I've more miles than I think).

I'm thinking the latter, that I'm logging more miles than odo shows.

I'm too stupid about this to figure which it really is.

TerribleTed

I think the radar speed sign is out of calibration.

draxcaliber

look at it this way, your speedometer and odometer are calulated on the rotation of your stock wheels and tires.

if you have bigger wheels now than what you had when you were stock, one rotation of a larger wheel will roll further than the distance of one rotation of your stock wheels.

so in one rotation, you are going to be going further, but your box will calculate it based on how far you would have gone on a stock tire size. so your speedometer should be actually going faster than what your speedo indicates. then your odo will follow suit and be reading lower than what your actual mileage is...i think i could be wrong... i just tried to ponder all this on my own just now.

MyCrisis

But it pegged my other car's speed dead-on twice, and gave the same "wrong" speed on the xB 3 times. If the gun were out of calibration then both car speedos would have read wrong vs the gun.

Weren't there postings about how 04 odos were off at one time?

Jayrock39

I've suspected that my speedo is off too. I haven't seen any radar signs to compare to, but I have noticed getting passed more in my xB, than in my wifes car, or in my old car, driving at the same speeds I usually do. And my xB is pretty much a brand new '06 that I picked up with only 30 miles on it.

ajayaitch

JDM spec bB's use 195/60-15 tires. Our USDM spec xB's use 185/60-15.
Tomas has posted about this stuff numerous times. IIRC, our odometer is off by about 3% one way, and the speedo is off by 3% the other.
Find a 10 mile stretch of highway, with markers, and you can figure it out yourself. Bring along a stopwatch too.

TerribleTed

Oops I didnt read your first post close enuf it seems (did not comprehend running the other car past it also). I would have to assume that either the xB (tires are just a touch small will read 1-2% fast) or the other car or the sign, or most likely all 3 are slightly off resulting in the differences. A measured mile at 60 on a marked highway is a better way to get an accurate feel for this (less variables).

pooder

That's not necessarily true. If everything is off because of tire size, it might be, but it could also be that the odometer is correct and the speedometer is off. For example, both my wife's and my BMW motorcycles show about 10% optimistic on the speedometers, and the odometers are spot on.

I haven't checked the xB yet, but I'm planning to on a trip this weekend. I'll ring in here on Sunday with my results (and another data point) if I remember . . . .

MyCrisis

Has anybody experience checking speed and distance with GPS?

10 mile stretch with markers? I get the idea, but there's lots o hills here that would make it tough to keep a steady speed. I'll try to find a flat marked mile, peg it at 60, and do the math then.

I measured a mile in my Honda, sticking pegs in the shoulder at end and beginning, and zeroing out the trip odo. Did the same in the xB.

Went about 120 feet further in the xB (didn't bring a tape measure - duh!). So the odo in the xB is underreporting my miles (if I trust my Honda's odo). One of them is off. I just don't know which!

I think GPS would solve this.

nameless_ak

My xB's way off for every 100 miles it shows only 93 miles, thats a pretty big difference.

kasra

I checked my mileage with a Nuvi 360 GPS today and got 77MPH while the speedometer indicated 80MPH. It also read 48MPH at 50MPH indicated. Over a ten mile distance, the odometer was off likewise by 3.75%. The best news is that my last tank was 36.5 MPG if corrected for the 3.75% discrepancy. I drive a '06.5 5speed.

OldYeller

I'm betting that the wheels+tires are a little different in size causing the difference in the ODO

kasra

My data was with OEM tires pumped to 40 PSI. I plan to go to 195's or 205's as soon as these wear out.

chadfo

I wouldn't assume the odometer is off. I put a Scanguage II in my car and the speed on it almost matched the roadside radars. I think I had to adjust it 1% so it would be dead on all the time. My speedometer normally runs about 3 MPH higher around 60. It's less at lower speeds and slightly more at higher speeds.

Dr_No

You are allow some error in the reading not sure what it is exactly. Honda had their odometer read too high. That was a problem for people who leased their vehicle and ended up paying excess mileage charges when they returned their cars at the end of the lease. The government asked Honda to refund the money for the excess mileage fee.

I think the Yamaha got caught with something similar with their R6. They claim the redline was 17500 but in actuality it was only 16000. They quietly refunded the people who bought the bike and complained about the misinformation.

MyCrisis

Boy, am I embarassed. Just bothered to search on this topic and found lots of posts about this already. Looks like I'm not the first.

Sorry to waste the bandwith. Thanks, all, for your input.

bB2NER

Considering the right stock size (bB from Japan) is 185/65/15 and you should have gotten 215/40/17's.
205/40/17's are too short and under weight rated for an xB.

MadMike550

somebody here helped me out by posting this...perhaps it could help out here??

Try here to see what you think
just start with the stock sizes and put in what your current setup is.

http://www.miata.net/garage/tirecalc.html